In a solar cell, carriers (electrons and holes) generated by light irradiation to a photoelectric conversion section composed of a semiconductor junction are extracted to an external circuit to generate electricity. Metal electrodes are provided on front surface and back surface of the photoelectric conversion section of the solar cell for efficiently extracting carriers generated at the photoelectric conversion section to the external circuit. For example, in a heterojunction solar cell in which a silicon-based thin-film is provided on a surface of a conductive single-crystalline silicon substrate to form a semiconductor junction, transparent electrode made of a transparent electroconductive oxide or the like is provided on a silicon-based thin-films on each of a light-receiving side and a back side, and metal collecting electrodes are provided on the transparent electrodes to collect photocarriers generated in the crystalline silicon substrate.
Light that is irradiated on regions where the metal collecting electrodes are formed is reflected on or absorbed by the metal collecting electrodes to cause shadowing loss. In order to reduce the shadowing loss, the collecting electrode on the light-receiving side is formed in a pattern shape. A typical example of the pattern of the collecting electrode is a grid pattern composed of finger electrodes and bus bar electrodes. The collecting electrode on the back side may be disposed on the entire surface of the back surface, or may be a pattern shape. In a solar cell module installed in such a manner that light enters into the module from back side as well as from front side, e.g., a flat roof type and on-ground installation type solar cell module, a patterned collecting electrode is provided on the back side of the solar cell. Also in a solar cell module having a structure in which light incident to an interspace between adjacent two cells is reflected on a back sheet, a patterned collecting electrode is provided on the back side of the solar cell.
The patterned collecting electrode is generally formed by screen printing of an electroconductive paste such as silver paste. The collecting electrode formed by using silver paste contains a resin material, so that it is high in resistivity and high in material costs. Methods for forming metal collecting electrodes by plating are proposed for reduction of the electrode material costs and the like. The plating method makes it possible to form a metal electrode large in thickness and low in resistance, so that the line width of the metal electrode is made smaller than the method using the electroconductive paste. Accordingly, formation of the metal collecting electrode by the plating method also has an advantage of improving the light collection efficiency of the resultant solar cell by decreasing a shadowing loss.
A method of forming a collecting electrode having a predetermined pattern by plating is known. In such a method, an insulating layer having openings is formed on a surface of a photoelectric conversion section and a metal is deposited on the surface of the photoelectric conversion section in areas where the openings are formed in the insulating layer. For example, Patent Document 1 discloses a method of forming an insulating layer having a thickness of about 10 to 15 μm on a transparent electrode of a photoelectric conversion section, making openings in the insulating layer, and then forming a collecting electrode by electroplating.
Patent Documents 2 and 3 propose a method of forming crack-like openings in an insulating layer deposited on the metal seed layer. In this method, a metal seed layer is formed by printing an electroconductive paste containing a low-melting-point material, and an insulating layer is formed thereon, followed by heating for annealing so that the low-melting-point material in the metal seed layer is thermally fluidized. This method is excellent from the viewpoint of material costs and process costs, since the method makes it possible to form openings selectively in the metal seed layer-formed region of the insulating layer, and thus any patterning of the insulating layer, such as using a resist and the like, is unnecessary.